The oxides of nitrogen herein referred to as NO.sub.x include nitric oxide, nitrogen dioxide and nitrous oxide. These oxides result primarily from the combustion of fossil fuels by automobiles and various stationary sources. It generally is believed that NO.sub.x is produced by nitrogen fixation wherein the oxygen combines with the nitrogen rather than fuel in the region of the "flame." NO.sub.x, and in particular NO.sub.2, undergoes photochemical reactions in the presence of hydrocarbons to produce smog. NO.sub.x is a deleterious air pollutant which can be poisonous at high concentration levels.
A number of methods have been introduced to reduce NO.sub.x caused by the automobile, e.g., U.S. Pat. No. 3,894,141, applicable to the automobile are applicable to stationary sources; however, the conversse is not generally true. The present invention is concerned exclusively with NO.sub.x reduction in stationary sources, and, in particular, the reduction in NO.sub.x emanating from electric power generating stations.
A significant body of technology has been developed in an endeavor to reduce or eliminate NO.sub.x from combustion process emissions. See, Gas Phase Decomposition of Nitric Oxide in Combustion Products, Muzio, Arand and Teixeira, 16th Symposium on Combustion 1971 and the EPRI Program on NO.sub.x Control Using Combustion Modification Techniques 2d Stationary Source Combustion Symposium 1978. Of particular interest is the state-of-the art survey of all known flue gas denitrification processes undergoing current development in the United States and Japan: Technical Assessment of NO.sub.x Removal Processes for Utility Application, Fawcett, Maxwell, and Burnett (EPRI/EPA Nov. 1977). [NTIS P B 276 637]. Forty-eight processes are reviewed, including the "DENOX" process of Exxon Research & Engineering Company which is a selective, noncatalytic process for reducing NO.sub.x by injecting ammonia into a flue gas having an excess oxygen content and a temperature below 2000.degree. F. More specifically, see U.S. Pat. No. 3,900,554 which describes the "DENOX" process as well as other known processes where ammonia is shown to be an effective reducing agent for NO.sub.x without the need of a catalyst.
In the DENOX process, a relatively large amount of ammonia is required for commercial application; for example, a NH.sub.3 : NO.sub.x mol. ratio of 3:1 to 4:1 is needed to obtain a 60 to 70% reduction in initial NO.sub.x. On the other hand, a better than 90% reduction is achievable by both catalytic processes and wet-type processes. In both prior art catalytic and noncatalytic processes excess air is required. While there has been a suggestion, EPRI Program on NO.sub.x, that reduction could be achieved in a fuel rich effluent, using methane, ethane and ammonia, such processes have not been used or found effective, see STATUS OF NO.sub.x CONTROL FOR COAL-FIRED POWER PLANTS, D. P. Teixeira. In fact, in the DENOX process it was found that NO.sub.x removal became insignificant in the absence of O.sub.2, Technical Assessment of NO.sub.x Removal.
The object of the present invention, therefore, is to provide a noncatalytic process for removing NO.sub.x wherein the removal is 80% or better with final NO.sub.x concentration of about 50 ppm or less by volume. It is also an object of the invention to provide a removal process that is compatible, either sequentially or simultaneously, with sulfur oxide removal processes. A further object of the invention is to provide a NO.sub.x removal process that is effective in combustion systems using any fossil fuel. The process of the invention is useful in both conventional boiler systems as well as in magnetohydrodynamic systems. These objectives and the advantages of the invention can be better understood by a perusal of the following description of the best mode of the invention presently contemplated taken together with the accompanying drawings.